The number of overweight and obese people (those with BMI above 25) around the world is approaching the two billion mark. This is more than 20% of estimated 7.4 billion people currently populating the planet. The connection between obesity and various chronic conditions such as cardiovascular disease, diabetes and some types of cancer is well established. Not much, however, is known about how the excess body weight influences the structure and function of brain.

Does IQ level determines body weight?

Statistically significant correlation between excess body weight and lower IQ level has been demonstrated in multiple studies. What was not clear for very long time is the direction of causality. Does the excess body weight cause the decline in intellectual capabilities? Or maybe people with lower IQ level are more prone to become overweight?

Although some earlier studies concluded that lower IQ level might be caused by obesity, the most recent prospectively longitudinal studies show that this is not correct. These studies demonstrate that one of the risk factors for obesity is lower IQ level.

A meta-analysis published in 2010 summarized 26 different studies on this topic. The main conclusion of this analysis was that there is a strong link between lower IQ level in childhood and the development of obesity in adulthood.

In one Swedish study involving 5286 males the IQ level was tested at the age of 18 and again at the age of 40. At each testing, the BMI of participants was also evaluated. The results clearly show that individuals with lower IQ level have higher BMI.

Another study performed in New Zealand included 913 participants. Their IQ levels were measured at the ages of 3, 7, 9, 11 and finally at the age of 38. This study also concluded that lower IQ level in childhood leads to obesity. People with lower IQ level at the age of 38 were more obese than people with higher IQ level.

Over 3000 people were participating in a study conducted in the Great Britain. The subjects were followed for more than 50 years. Their IQ levels were measured at the age of 7, 11 and 16. At the age of 51, their BMI was measured. Their results show without any doubt that IQ level at the age of 7 can predict higher BMI at the age of 51. Also, the results show that BMI grows faster after the age of 16 among people with lower IQ level.

Another study conducted in the Great Britain involved 17,414 individuals. The IQ level was assessed at the age of 11. BMI was evaluated at the ages of 16, 23, 33 and 42. The results of this study also confirm that lower childhood IQ level leads to obesity in adulthood.

Obesity leads to faster aging of brain

Our brain changes during the natural aging process. As we become older, the brain loses white matter and shrinks. But the rate of aging process is not the same for every person. Individual factors may lead to faster or slower age-related brain changes. One of these factors that affects our brain structure is excess body weight. Obesity alters the normal aging process by speeding it up.

Research study conducted at the University of Cambridge concluded that obese people have less white matter in their brain compared to normal weight individuals. The brain structure of 473 individuals was investigated in this study. The data showed that the brain of obese people appears to be up to ten years anatomically older in comparison with the normal weight counterparts.

Another study conducted on 733 middle aged individuals showed that obesity is strongly linked with the loss of brain mass. Scientists measured body mass index (BMI), waist circumference (WC), waist-to-hip ratio (WHR) of participants and used brain MRI to find and identify the signs of brain degeneration. The results demonstrated that brain degeneration is more extensive in people with higher BMI, WC, WHR than in normal weight persons. The scientists hypothesize that this loss of brain tissue may lead to dementia, although there are no hard proofs at present.

Obesity changes the way we feel

Apart from structural changes, obesity can also change the way our brain works. Dopamine is one of the neurotransmitters which is involved in reward circuits and motivation. One study concluded that concentration of available dopamine receptors in the brain is in correlates with BMI. Individuals with higher BMI have a lower concentration of available dopamine receptors that may lead to a lack of pleasure after eating normal size portions and the urge to eat more to feel satisfied.

This view was confirmed by another study which analyzed the response of obese people to milkshakes during a period of time. Their response was analyzed using functional MRI. The measurements were repeated half a year later and showed that brain response was a lot weaker in people who gained excess body weight between two measurements. The researchers concluded that obese individuals feel less satisfaction when eating in comparison to lean individuals, due to a lower concentration of dopamine receptors in the brain.

The research on the effects of obesity on brain functions are still in infancy but the findings described above are already alarming enough. I think it is important to raise the public awareness about this issue. The negative impact of obesity on general health is well publicized, but hardly anyone ever mentions how bad the excess body weight can be for our cognitive functions.

Viatcheslav Wlassoff, PhD, is a scientific and medical consultant with experience in pharmaceutical and genetic research. He has an extensive publication history on various topics related to medical sciences. He worked at several leading academic institutions around the globe (Cambridge University (UK), University of New South Wales (Australia), National Institute of Genetics (Japan). Dr. Wlassoff runs consulting service specialized on preparation of scientific publications, medical and scientific writing and editing (Scientific Biomedical Consulting Services).

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